Transformer with variable secondary reactance

ABSTRACT

Single phase or three phase welding transformers having a primary coil wound on one limb of a rectangular core, a secondary coil wound around both said one limb and a movable core member so as to enclose said primary coil. The limbs of said rectangular core adjacent said one limb are formed with upper and lower extensions respectively which abut the upper and lower ends respectively of said movable core member when in its lowered position. The movable core member is movable in a direction substantially parallel to the longitudinal axis of said secondary coil such that an air gap of variable width is formed between the lower extension and the lower end of the movable core member. Non-magnetic guides fixed to the movable core member are spring urged against a bearing surface provided on a casting fixed to the rectangular core. The reaction to the spring force is also provided by the casting. Gaps may be maintained at all times between the rectangular core and the movable core member and/or bias coils provided to maintain high open circuit secondary voltage.

United States Patent Hirst [is] 3,686,464 [451' Aug. 22, 1972 541 TRANSFORMER WITH VARIABLE SECONDARY REACTANCE I [72] lnventor: Sidney Hirst, Gatwick Rd., Crawley,

England [22] Filed: Aug. 13, 1970 [21] Appl. No.:' 63,412

1 Related us. Application Data [63] Continuation-impart of Ser. No. 749,050,1u1

31, 1968, abandoned.

[30] Foreign Application Priority Data Aug. 21, 1967' Great Britain ...38,465/67 [52] US. Cl ..2l9/131 R, 2l9/135, 336/l34 [51] Int. Cl. ..B23k 9/10 [58] Field of Search ..2l9/l3l, 135; 336/10, 130, 336/132, 133, 134, 216; 323/48, 51, 89 MS [56] References Cited UNITED STATES PATENTS 7 2,411,370 11/1946 Fries ..336/133 2,591,582 4/1952 Monette ..336/l34 X 864,257 l/l94l France "336/133 356,487 1931 Great Britain ..336/134 Primary Examiner-J. V. Truhe Assistant Examiner-George A. Montanye Att0rney-Malcolm W. Fraser ABSTRACT Single phase or three phase welding transformers having a primary coil wound on'one limb of a rectangular core, a secondary coil wound around both said one limb and a movable core member so as to enclose said primary coil. The limbs of said rectangular core adjacent said one limb are formed with upper and lower extensions respectively which abut the upper and.

lower ends respectively of said movable core member when in its lowered position. The movable core member is movable in a direction substantially parallel by the casting. Craps may be maintained at all times between the rectangular core and the movable core 3,059,170 10/1962 Jetter... ..219/ 131 R member and/or bias coils provided to maintain g FOREIGN PATENTS OR APPLICATIONS p n Circuit c r y Voltage.

325,441 3/1935 ltaly ..336/133 20 Claims, 14 Drawing Figures 1 77d 7 4 v I /8 /2 g Pmmd Aug. 22, 1972 3,686,464

7 Sheets-Sheet 1 [ELECTRODE FIG' SUPPORT 706 ELECTRODE me s l ll/0 SUPPLY""/' 1/4 712 04 E 0 0 W wo KPIECE C F IG2.

FIGB.

7/6 PRIOR ART V A PRlOR ART INVENTOR SIDNEY HIRST ATTORNEYS Patented Aug. 22, 1972 7 Sheets-Sheet 2 I I I I I ATTORNEYS Patented Aug. 22, 1912 3,686,464

1 7 Sheets-Sheet 3 FIG .6.

MAX

PRIOR ART FIGB.

PRIOR ART INVENTOR SIDNEY HIRST ATTORNEYS Patented Aug. 22, 1972 3,586,464

7 Sheets-Sheet 4 FIGS. I

INVENTOR SIDNEY HIRST ATTORNEYS Patented Aug. 22, 1972 7 Sheets-Sheet 6 INVENTOR SIDNEY HIRST MM ATTORNEYS Patented Aug. 22, 1972 7 Sheets-Sheet 6 FIG".

INVENTOR SIDNEY HIRST ATTORNEYS TRANSFORMER WITH VARIABLE SECONDARY .REACTANCE This is a continuation-in-part of US. Patent application Ser. No. 749,050 filed July 31 1968, now aban- Electric arc welding processes are known in which a result of which high current flows throughthe electrode and the workpiece and causes heating of the electrode to occur. When the electrode is withdrawn from the workpiece, the required arc is struck.

It is well known that to prevent exceptionally high current flows occurring through the electrode and the workpiece when the electrode is moved into contact with the workpiece, the voltage which is applied should droop to substantially zero during the period in which the short circuit occurs i.e,. while the electrode is in contact with the workpiece. When the electrode is withdrawn in order to strike the arc, the voltage applied should increase again in order to maintain the arc ignited. However, the value to which the voltage rises when the arc is struck should be less than the initial or open circuit value of the voltage before the electrode is moved into contact with the workpiece.

It is furthermore desirable to be able to achieve different magnitudes of current flow both while the arc is alight and during theshort circuit period prior to striking of the arc. This adjustment is desirable to take account of various factors, such as different size electrodes.

In electric arc welding equipment in common use the above described droop characteristic for the voltage applied across the welding electrode and the workpiece and the above described adjustment are provided by the use of a transformer specially designed for the purpose. Such transformers essentially involve a variable reactance magnetic path.

The transformers which are in common use suffer thereof, in which the structure and advantages of the v invention as compared to the prior art transformers are discussed.

In the accompanying drawings:

FIG. 1 is a diagram, essentially in block form, of a welding apparatus of the type to which the invention may be applied;

FIG. 2 is a graph showing how the voltage droops with increasing welding current in the apparatus of FIG. 1;

FIG. 3 is a diagram of a typical form of transformer currently in common use in the welding apparatus of FIG. 1;

FIG. 4 is a graph showing the voltage against current characteristics of the transformer .of FIG. 3 and also illustrating the degree of adjustment available;

FIG. 5 is a diagram showing the principle parts of the transformer according to the present invention;

FIG. 6 is a graph illustrating the voltage against current characteristics of the transformer of FIG. 5 and showing the degree of adjustment available;

FIGS. 7 and 8 are graphs illustrating the voltage against current characteristics of two other forms of transformer which have been proposed in the prior art;

FIGS. 9 to 12 are respectively a side view, a front view, a plan view partly in section and a detail of a portion of a practical embodiment of the transformer of the present invention; and

FIGS. 13 and 14 are respectively a plan view and a side view of three interconnected transformers accord- 1 ing to the invention arranged for use with a three phase supply.

Referring to FIG. 1, a workpiece 100 to be welded is connected by a lead 102 to one end of the secondary winding 104 of a transformer 106. The other end of the secondary winding 104 is connected via a lead 108 to a consumable electrode 110 to be used in the welding operation. The electrode 110 is carried in a conventional support 111. The electrode 110 may, for example, have a coating of flux upon it. The primary winding 112 of the transformer 106 is connected to an AC from the disadvantages-that a wide degree of adjustv degree of adjustment of secondary current without any a substantial variation in the open circuit voltage developed by the secondary, and in which close coupling between the primary and secondary coils enables high values of power output to be obtained.

. The essential features and the advantages of the invention will be clearly understood by reference to the accompanying drawings and the following description mains supply 114.

The welding operation is carried out by establishing an arc between the electrode 110 and the workpiece 100. The are is established by firstly moving the electrode 110 into contact with the workpiece so that a short circuit is formed across the secondary winding 104 of the transformer 106. This short circuit causes a cordingly, the transformer 106 is so designed that when r the short circuit occurs across the secondary winding 104 by contacting the workpiece 100 with the electrode 110, the voltage across the secondary winding 104 drops substantially to zero. Thus, as seen in FIG. 2, in the open circuit condition Thus, as seen in FIG. 2, in

the open circuit condition before the electrode 110 is moved into contact with the workpiece 100 the voltage V across the secondary winding 104 has a value indicated by the character A, and the current is zero. When the short circuit occurs, current flows through the electrode 110 and the workpiece 100 but as the current increases in magnitude the voltage across the secondary winding 104 droops until, at point B on the curve in FIG. 2, the voltage is zero and the current has reached a predetermined maximum value. Upon withdrawal of the electrode 110 from the workpiece 100 to establish the arc, the voltage across the secondary winding 104 again increases and the current decreases until, typically, while the arc remains ignited the voltage and current assume values represented by the point C on the curve in FIG. 2. In practice, the voltage and current will oscillate between points A and C at the frequency of the AC supply 114, the arc being momentarily extinguished each time the current becomes zero at point A.

When welding under different conditions, for example with electrodes of different diameter or with workpieces of different size, it is desirable to provide for adjustment of the values of the current as represented by the points B and C in FIG. 2. For example, with thinner electrodes it may be desirable to shift the point B of FIG. 2 to position B and the point C to position C.

The structure of a commonly used transformer for achieving the voltage droop characteristic of FIG. 2 and for enabling adjustment of the points B and C is illustrated in FIG. 3. This transformer comprises a rectangular core 116 on one limb of which primary and secondary coils 112g and 1044' are provided at spaced positions. On the opposite limb further primary and secondary coils 1121 and 10411 are provided also at spaced positions. The primaries 112g and 11211 are connected in series with one another, as are the secondaries 104a and 10411.

In the spacing between the primaries and secondaries, a movable iron member 118 is arranged. The direction of movement of the-iron member 118 is perpendicular to the plane of the paper. The iron member 118 extends substantially between the opposite limbs on which the primary and secondary coils are mounted.

When the iron member 118 is inserted between the limbs on which the primary and secondary coils are mounted, magnetic flux produced in the core 116 by the primary coils tends to be diverted through the iron member 118 rather than through the secondary coils 104. This produces a relatively low voltage and current output from the secondary coils. The voltage and current output from the secondary coils can be increased by withdrawing the iron member 118 from between the limbs of the core 116 so that less flux is diverted through the member 118. In this way, the points B and C on the curve of FIG. 2 can be adjusted. However, in so adjusting the points B and C the point A is also adjusted. Thus, when the iron member 1 18 is in, while the points B and C may be adjusted to the points B and C the point A also moves to the position A. That is to say, the open circuit voltage developed by the secondary windings of the transformer is reduced in magnitude. This is undesirable since reduction in the open'circuit voltage makes it difficult to maintain the arc alight when the points B and C are adjusted to low values. It

will beappreciated that in each cycle of the AC current in the secondary circuit the arc is effectively extinguished at the point of zero current. In order for re-ignition of the arc to occur it is necessary that the voltage at this time should be high. Thus, since in the transformer of FIG. 3 the open circuit voltage across the secondary winding of the transformer reduces when the points B and C are adjusted to low values, the degree of adjustment which can be effected in practice is severely limited.

Furthermore, the maximum value of the power output which can be obtained with the transformer of FIG. 3 is unduly limited since the primaries 112g and 112k are spaced apart from the secondaries 104;; and 1041; so that the flux. linkage between the primary and secondary coils is relatively poor.

Despite these problems, the transformer of FIG. 3 has been in common use for many years since, prior to the present invention, a practical alternative form of transformer which will give high power output and wide degrees of adjustment of the output current without reducing to any substantial extent the open cirprises a core 11 having a center limb 12, an end limb 13 r and upper and lower cross members 14 and 15 which include extensions 16 and 17 respectively. Vertically displaceable, adjacent to an end of the extension 16 FIG. 5, so that after initial vertical upwardsmovement of the member 18 from its rest position in which it contacts the extension 16, no contact occurs thus preventing wear on the two components. It will be understood that the core 11 and member 18 are built up from laminations stamped out of sheet material. The core 11 is fabricated in two parts 11;; and 1111 which are joined by V shaped joints 21 and 22, in the limbs 12 and 13 respectively. This construction enables the transformer coils to be positioned on the part 1111 of the core prior to connecting the two parts 11g and 1 lb together. The whole core assembly 11 is rigidly clamped in a frame so as to hold the two parts 11g and 1113 firmly together. Although forming the core in two parts results in an increase in the reluctance of the magnetic path around the core, the V shaped joints 21 and 22 are such that they present an area for the flow of magnetic flux across the air gap which necessarily is present between the two parts that is approximately twice the cross sectional area of the respective limbs 12 and 13 so that the reluctance is kept to a minimum.

As can be seen the limb 13 together with the cross members 14 and 15 are wider than the limb 12 and the movable core member 18. The depth of the core 11 is uniform throughout and equal to the depth of the member 18. A primary winding 23 of the transformer surrounds only the center limb 12 of the core; and a secondary winding 24 surrounds both the primary winding 23 and the movable core member 18. The smaller dimensions of the limb 12 and the member 18 around which the transformer coils are wound permit the dimensions of the coils to be kept low thus reducing the amount of copper needed in these coils. Further, the larger dimension of the limb 13 and members 14 and 15 provide anexcess flux path of low reluctance for the flux from the primary, which enables the primary current under open circuit conditions of the secondary coil to be kept low. V

A portion 26 of the lower limb 15 is narrowed to substantially the same dimension as the width of the limb 12, and an auxiliary coil 27 is wound in this narrowed portion. Current is supplied to the coil 27 in such a direction that the flux produced thereby opposes the flux which would otherwise follow a path through the core 18. The coil 27 is shown as connected to a tapping on the primary coil 23 for the supply of current but alternatively may be connected to the whole or a portion of thecoil 24. A second auxiliary coil 28 is also wound in the narrowed portion 26 and supplied with current by being connected to the secondary coil 24. The direction of current flow in-the second auxiliary coil 28 is such that as soon as current is drawn from the secondary coil 24, the flux induced by the second coil 28 opposes the flux induced by the first coil 27. Conveniently the second coil 28 is in series with the secondary coil 24. In the case of a welding transformer where the secondary coil 24 consists of a few turns of thick copper wire, the coil 28 can be in the form of a single turn of this thick wire.

The voltage/current characteristics of the transformer of FIG. 5 are illustrated in FIG. 6. The open circuit voltage is again indicated by the character A. When the movable core member 18 is withdrawn from the secondary coil 24, the characteristic curve is the one marked MAX in FIG. 6. Thus, when the welding electrode is brought into contact with the workpiece, a relatively high current flows through the electrode as indicated by the point B. When the electrode is withdrawn to strike the arc, the welding current assumes a value indicated by the character C, this value still being relatively high. d

When the movable core member 18 is fullyinserted into the secondary coil 24, the short circuit current when the electrode is in contact with the workpiece and the current which flows when the arc is established are, as indicated by points B and C on the curve marked MIN, much lower than when the movable core member 18 is withdrawn. It will be seen that the points B and B and C and C are widely spaced indicating that a wide degree of adjustment of current is available with the transformer of FIG. 6. Furthermore, it will be seen that the point A indicating the open circuit voltage which is present when the current through the electrode is zero remains at a constant level. Maintenance of the open circuit voltage at a constant level is assisted by the provision of the biassing coil 27 which, in the absence of, secondary current, opposes the flow of flux through the core member 18. This biassing coil 27 even makes it possible to cause the open circuit voltage to rise when the movable core member is inserted into the secondary coil 24, by increasing the number of turns in the coil 27 or increasing the magnitude of the current which flows through the coil 27.

It should further be understood that with the structure of transformer illustrated in FIG. 5 it is possible to dispense with the biassing coils 27 and 28 without causing an unacceptable lowering of the open circuit voltage when the core member 18 is fully inserted into the secondary coil 24. If the biassing coils 27 and 28 are omitted, however, additional reluctance against flux flow through the core member 18 should be provided, as by providing that a gap exists between the extensions 16 and 17 andthe member 18 even when fully inserted, to inhibit the flux flow through the core member 18 when it is fully inserted; the narrower dimension of the core member 18 relative to members '13, 14 and 15 contributes to this. As an example, it is possible by these means, but with the bias coils 27 and 28 omitted,

to maintain the open circuit voltage when the core derstood that this voltage drop, of 7 to 8 percent, is'

substantially less than that (typically 20 percent) which occurs in the transformer illustrated in FIG. 3, as shown by the difference between the points A and A in FIG.

v 4. It is preferred that the voltage drop in the present invention does not exceed 10% since exceeding this limit tends to reduce the versatility of the transformer.

It is actually preferred in some applications of the invention that even when the bias coils 27 and 28 are pro vided, gaps should be present between the core member 18 and each of the extensions 16 and 17 when the core member 18 is fully inserted into the secondary winding 24. This makes it easier to carry out movement of the core member 18 to and from its fully inserted position without switching the transformer off and even during the time that current is being drawn from the secondary winding 24. If no such gaps were provided,

the core member 18 could not be moved so easily from its fully inserted position under load conditions and unless the transformer were switched off, in view of the magnetic forces which would tend to retain the core member 18 in contact with the extensions 16 and 17. It

. should be emphasized, however, that the effective line contact provided by the notch and rib 19, 20, minimizes the forces, as does the fact that the contact, if it occurs between member 18 and extension 16, is such that the forces are almost at right angles to the direction of movement of core 18.

It should further be noted that the provision of the notch 19 and corresponding-projection 20 on the extension 17 ensure that a smooth increase in the reluctance of the gap between the core member 18 and the extension 17 takes place. Providing simply plane surfaces at this point would result in a rather sharp increase in reluctance, making a fine selection of low values of current difficult to achieve, as well as high magnetic forces as referred to above.

It may be desired in some instances to provide a rather higher current through the electrode under short circuit conditions than indicated by the points B and B in FIG. 6 without afi'ecting the welding current indicated at points C and C. For example, it may be desired that under short circuit conditions the current should be as indicated by the point B1 in- FIG. 6-i.e. a

higher value than the current at point B, when the core member 18 is withdrawn, and that the short circuit current should assume a value at for example B1 when the core member 18 is fully inserted. This modification of the characteristics of the transformer can be achieved by passing a small number of turns of the secondary coil, for example one or two turns, inside of the movablecore member 18 as indicated by the reference number 24;; in FIG. 5. The degree to which the characteristics are modified depends upon the number of turns of the secondary coil passed inside the movable core member 18 at 24g.

It has been discovered, further, that the passing of such a turn of the secondary coil inside the movable core member results in the advantage of a reduction in stray flux losses, thus reducing vibration in the transformer and the surrounding structure and rendering the transformer more efficient.

It will be understood from the above discussion that the characteristics of the transformer according to the invention are a substantial improvement over the characteristics of the transformers which have been in common use in welding for many years.

The substantial advance in the art provided by the present invention, and the difficulties in advancing this art, will be more fully appreciated from the following discussion of certain other proposals which have been made in the prior art but which are believed never to have been put into practice. First, reference is made to Italian Pat. No. 325,441 which illustrates in its FIG. 2a transformer which at first sight appears somewhat similar to the transformer of the present invention. However, the transformer of the Italian Patent includestwo sets of primary and secondary coils, only one of which is provided with a variable reactance within the secondary winding. The Italian Patent suggests a number of uses for the transformer which is described, one of which is said to be in electric welding. The characteristics of the transformer of FIG. 2 of the Italian Patent are illustrated in 'FIG. 7 of the accompanying drawings. In FIG. 7 the curve MAX represents conditions when the movable core is out, and MIN when it is in. It is seen that whether or not the movable core member is in or out the voltage only droops by a small amount under short circuit conditions. In other words, under short circuit conditions a very high current will flow since as seen at the point marked X on the graph of FIG. 7 the rate of decrease .of voltage with increasing current becomes very small indeed. Hence, the transformer of the Italian Patent is not suitable for welding operations in which drooping of the voltage to zero, to avoid excessive flow of current, is desired.

Another form of transformer which has been proposed but is believed never to have been used is illustrated in French Patent No. 864,257. The variable secondary reactance in the French Patent is provided by two L shaped members, one at each end of the coils, and each being pivotally supported at the end of one leg so that its other leg projects into a space between the primary and secondary coils which are wound around one another. A block of iron is permanently fixed in the space between the primary and secondary coils. FIG. 8 shows the characteristics.

The substantial range of adjustment available .in the present invention cannot be achieved in the proposed transformer described in the French Patent, as indicated by the closer spacing between the MAX and the primary and secondary coils. In the present invention, substantial elimination of flux leakage is achieved by effectively completely withdrawing the movable core 18 from the secondary coil 24; but in the proposed transformer of the French Patent the block of iron per manently fixed in the space between the primary and secondary coils means that substantial elimination of flux leakage cannot be achieved. The requirement of close magnetic coupling between the primary and secondary coils is achieved in the present invention since the movable core member 18 is withdrawn from and inserted into the gap between the primary and secondary coils substantially along a straight path so that the spacing between the primary and secondary coils to accommodate the core 18 is of minimum dimension; in the proposed transformer of the French Patentthe movable core members pivot and to allow for this pivoting the gap between the primary and So far as low values of welding current are concemed, it has been explained previously that high open circuit voltage must be maintained to ensure that the arc is re-ignited following each zero point in the AC cycle. The present invention achieves this high open circuit voltage even with low values of current by means of the biassing coils'27 and 28 and/or by the provision of gaps between the movable core member 18 and the extensions 16 and/or 17 even when the movable core member is fully inserted. Whatever the means for retaining the open circuit voltage at a high value when the core is fully inserted, it is necessary that under short circuit conditions and during the period while the arc is alight, leakage flux should flow through the secondary winding relatively freely so as to limit the short circuit current and the welding current to the low values desired. In the invention, in the case when the biassing coils 27 and 28 are provided, the biassing coil 28, in opposing the effect of the coil 27, ensures the free flow of leakage flux atthis time; and in the case where the bias coils 27 and 28 are not provided the reluctance in the flux leakage path provided by the core 18 is kept to a small value as at most two gaps, namely between the core member 18 and the respective extensions 16 and 17, are present, the small value of the reluctance nevertheless being sufficient to ensure that the open circuit voltage remains at a reasonably high value as explained previously. In the French Patent, however, the flux leakage path through the secondary winding involves four air gaps, two between the respective opposite ends of the fixed block of iron within the secondary coil and the ends of the arms of the L shaped movable core members projecting into the secondary coil and another two between the other arms of the L shaped core members and the core on which the primable in the French Patent are higher than in the inven-- tion. Of course, attempts could be made to reduce this bolts 220 and held steady in an upright position by a reluctance by reducing the size of the gaps but, the

smaller the gaps the higher. thealternating magnetic forces and therefore the more difficult it is to avoid vibration of the transformer; and complete elimination of the gaps would result in an unacceptable drop in the open circuit voltage of the transformer when adjusted for minimum welding and short circuit currents. It is to be noted that there is no provision in the transformer proposed by the French Patent for preventing such vibration.

Further typical proposals which have been made in the prior art are-illustrated in British Pat. No. 580,600 and US. Pat. No. 2,591,582. Each of these proposals involves a pair of primary and secondary coils, each secondary being provided with a magnetic flux leakage path of variable reluctance. It is believed that none of these proposals has ever been put into practice. The basic problem which arises is that severe vibration results from the alternating magnetic forces which act upon each of the movable core members and this produces opposing forces on the supporting structure for the moving members. No successful means for balancing out these opposing forces has been found. Also, each of these prior proposals provides for the location of iron fixed within the gaps between the primary and secondary coils as a result of which the degree of control available is unduly limited. These factors thus render the proposals impracticable.

The preferred practical form of applicants invention will now be described with reference to FIGS. 9 to 12 in which the same reference numerals as used in FIG. 5 are used to designate corresponding parts of the transformer.

The two parts 11g, 11g of the core 11 are secured together at the limb 12 by a nut and bolt'200 and at the limb 13 by a pair of plates 202 overlying the joint 22 and fixed in position on the respective opposite sides of the limb 13 by bolts 204 and 206 which respectively pass through the upper end lower parts 11g and 11b. The joint 22 at the limb 13 of the core is further reinforced by a plate 208 which is welded in position overlying thejoint 22.

The movable core member 18 is carried by a struc ture 210 which is secured to the upper cross member 14 of the core 1 1. The structure 210 comprises a pair of rigid supporting castings 212, for example of a suitable steel, which are secured to the upper cross member 14 by bolts 214 and 216 passing through the upper cross member 14. A film of electrically insulating material (not visible in the drawings) is provided between the castings 212 and the core 11 when contact would otherwise occur. A bridge 218 is fixed. to the casting 212 the latter bolt 216 carrying spacers 217 between the castings and the cross member 14. The castings are pegged to aluminum L-pieces 2l3'which are in turn pegged to the member 14 to inhibit relative sliding movement between the castings and the member 14 by strut 222 fixed at one end to the bolt 216 and fixed at its other end to the bridge 218 by bolts 224. Secondary coil support members 229 of non-magnetic material, preferably hard-board or the like, are carried by the bridge 218 and the lower limb extension 17. Terminal boards 231, for associated terminals and circuitry such as rectifiers are also carried by the bridge 218.

A bearing 226 is fixed to the cross member of the bridge 218 and rotatably carries a hollow internally threaded shaft 228 to the top end of which a crank 230 is fixed for rotating the shaft 228. An externally threaded shaft 232 has its upper end threaded into the hollow shaft 228 and its split at its lower'end at 232g.

The split 232g embraces a pin 234 and isattached thereto by a further pin 236 which extends through transverse apertures in' the split end 232g of the shaft 232 and in the pin 234. The pin 234 is itself attached to a channel shaped steel bracket 238 which is welded to the top of the moving core member 18. In this way, the moving core member 18 is suspended from the shaft 232 and can thus be raised and lowered by rotating the crank 230.

Brass guides 240 of channel section are secured to opposite sides of the movable core member 18 and are disposed within recesses 212g of the castings 212. Leaf springs 242 act through bearing blocks 244 of silver steel to press the guides 240 against one end face 212k" of the recesses 212g. The bearing blocks 244 are loosely mounted on the castings 212 by bolts 246 which pass through slots or elongated openings 248 in the castings 212. The springs 242 react against studs 250 which are threaded into end portions 212; of the castings 212 and I carry lock nuts 252. Loosening the lock nuts 252 and rotating the studs 250 causes the studs to move in one direction or the other thus adjusting the pressure applied by the springs 242. Thus, wear between the guides 240 and the bearing blocks 244 and surface 212k can be taken up by adjustment of the stud 250. Lubricant may be provided on the surface to facilitate sliding and inhibital wear. Displacement of the movable core member 18 arising from such wear is pennitted by the loose connection between the core member 18 and the shaft 232, which connection provides for both free pivoting of the pins 234 andi236 and also axial sliding of the pin 236 relative to the bracket 238.

As more clearly shown in FIG. 5, the core member 18 has its surface 18g adjacent to the extension 16 somewhat tapered so that there is a small gap between the end of the extension 16 and the core member 18. This can also be seen in FIG. 12. Such a taper is not, however, provided on the guides 240. Rather, the guides 240 and the surface 212g are so arranged that even when the movable core member 18 is in its lowermost position a small gap is present between the core member 18 and the extension 16. Also, the extent of member 18. Thus, since the movable core member 18 never actually comes into contact with the core 11, the

high magnetic adhesion which would take place due to such contact when the transformer is in operation is avoided. Hence, it is possible to raise and lower the v movable core member 18 when the transformer is in operation and without interrupting the supply of current to the primary coil 23. Further, the mounting of the movable core member 18 in this way by the utilization of guides which prevent the core member 18 pressing against the fixed core 11 avoids wear on the core members and ensures that the transformer has a long life and, furthermore, it makes it possible to construct the transformer without the necessity for high tolerances. The use of the springs 242, in that they act to press the guides 240 secured to the core member 18 against the casting 212 and also have'their reaction forces provided by the casting 212, namely the end portion 212g thereof, ensures that vibration of the core member, due to the alternating magnetic forces between the core member 18 and the core l1,'may be substantially eliminated. Such vibrational forces have hitherto been a serious problem in the construction of transformers but this problem is solved by applicants invention in a transformer which is nevertheless compact and economical to manufacture.

The transformer shown in FIGS. 13 and 14 of the accompanying drawings is a three-phase equivalent of the single-phase transformer shown in FIG. 5 or in FIGS. 9 to 12. In essence, it comprises three of the single-phase transformers, set at 120 to one another and each with its core limb 13 omitted, the corresponding ends of the three cross members 14 being magnetically continuous with one another and the corresponding ends of the means are not shown in FIGS. 13 and 14. The compact configuration thus provided, as best seen in FIG. 13, is the more advantageous in that within its general outline it can accommodate three rectifier units 29 which may be included in the secondary circuit of a welding transformer.

As shown in FIG. 14 by chain lines 18', the core members 18 will project substantially above the top of the cross members 14 when raised to their maximum extent, but this does not mean that the transformer requires space to be left above it which would otherwise be wasted; for transformers of this kind require in any case to be cooled by forced ventilation, and conveniently a fan for this purpose is mounted in the space above the cross members 14, within a region indicated by chain lines 30 in FIG. 13.

Although in the foregoing description it has been assumed that the welding current is'AC,-it is to be understood that the invention is also applicable to situations in which a rectifier is connected to the secondary winding of the transformer to provide DC welding currents. Rectification, even with smoothing, does not remove the requirement for substantially constant high open circuit voltage.

What I claim is:

1. Electric arc welding apparatus comprising means for supporting a welding electrode adjacent to a workpiece to be welded, said support means permitting said electrode to be moved into contact with said workpiece and withdrawn therefrom for establishing an arc between the electrode and the workpiece; and power supply means for supplying current and voltage to said electrode and said workpiece, said power supply means being operable such that when said electrode is in contact with said workpiece thereby establishing shortcircuit conditions the voltage applied drops to substantially zero thereby to limit the maximum value of said short circuit current to a predetermined magnitude and when said electrode is withdrawn from said workpiece to establish said arc the voltage supplied increases from said zero value to a value less than the value under open circuit conditions to provide a welding current of predetermined magnitude, said power supply means further including adjusting means for selecting different values of said currents; said power supply means comprising a transformer which includes:

a generally rectangular core having first and second limbs spaced from one another and third and fourth limbs spaced fromone another and interconnecting adjacent ends of said firstand second limbs, said first, second, third and fourth limbs providing a closed main flux path having no gaps therein;

a primary coil which surrounds said first limb of said rectangular core;

a secondary coil which surrounds said primary coil and is operable to produce said voltages and said currents;

said adjusting means comprising a movable core member displaceable substantially rectilinearly in a direction parallel to said first limb between a first position in which said movable core member is substantially completely removed from a space defined between said primary and secondary coils and a second position in which said movable core member is within said space and extends at least between opposite ends of said secondary coil, and extension limbs magnetically coupling opposite ends of said movable core member to opposite ends of said first limb and said movable core member is in said space, said extension limbs and said movable core member providing through said space a flux leakage path whose reluctance is variable by varying the position of said movable core member; and

means for inhibiting the flow of flux in said leakage path, at least when said movable core member is so positioned within said space as to provide low magnitudes of said welding current, to at least prevent the magnitude of said voltage under open circuit conditions from dropping by more than a predetermined amount;

said transformer having no further primary and secondary coils and no further limbs and no further movable core members.

2. Apparatus according to claim 1, in which said flux inhibiting means comprises a biassing coil arranged to produce in said flux leakage path a flux which opposes flux from the primary coil.

3. Apparatus according to claim 2, including an auxiliary coil operable when current is drawn from said forces between said movable core member and said exsecondary coil to substantially cancel the, effect of the flux from said biassing coil.

4. Apparatus according to claim 3, wherein said biassing coil is connected to the primary coil and supplied with current therefrom, and the auxiliary coil is connected to the secondary coil and supplied with current therefrom. l

5. Apparatus according to claim 3, wherein said biassing coil is operable to produce a flux which maintains said voltage under open circuit conditions substantially constant for all positions of said movable core member.

6. Apparatus according to claim 3, wherein said biassing coil is operable to produce a flux which increases the magnitude of said voltage under open circuit conditions the further into said space that the movable core member is positioned.

7. Apparatus according to claim 1, wherein said flux inhibitingmeans comprises a gap between said movable core member and at least one of said extension limbs when the movable core member is fully inserted between the primary and secondary coils, said gap being so dimensionedthat the magnitude of said voltage under open circuit conditions drops by not more than percent when the movable core member is fully inserted into said space as compared to the magnitude of said voltage under open circuit conditions when the movable core'member is completely removed from said space.

8. Apparatus according to claim 1, wherein said flux inhibiting means comprises a gap between said movable core member and each of said extension limbs when the movable core member is fully inserted between the primary and secondary coils, said gaps being so dimensioned that the magnitude of said voltage under open circuit conditions drops by not more than 10% when said movable core member is fully inserted into said space as compared to the magnitude of said voltage under open circuit conditions when. the movable-core member is completely removed from said space.

9. Apparatus according to claim 1, wherein one side of said movable core member is tapered and an adjacent one of said extension limbs has an end surface presented towards said tapered side of said movable core member, said end surface being correspondingly tapered such that a gap of increasing width is provided between said end surface and said one side of said movable core member upon withdrawal of said movable core member from said space.

10. Apparatus according to claim 1, wherein said movable core member has an end face presented towards a side face of one of said extension limbs, one of said faces being provided with a recess and the other of said faces being provided with a projection arranged for location within said recess when the movable core member is fully inserted into said space, said projection and recess providing for smooth adjustment of the magnitude of said short circuit current and said welding current when said magnitude is near to the 'minimum available and providing for reduction in magnetic tension limb when saidmovable core member is in the region of its fully inserted position.

11. Apparatus according to claim 1, wherein at least 0 e but not more an a small number f the turns of sa id secondary coil i s located between saiii primary coil and the path of movement of the movable core member to provide for all positions of said movable core member relatively larger short circuit currents substan-- tially without affecting the magnitude of said welding currents.

12. Apparatus according to claim 1, including a rigid support fixed against movement relative to said rectangular core, means mounting said movable core member on said rigid support, co-operating bearing means p'rovided on said rigid support and said movable core member, said bearing means permitting said movement of said movable core member, andresilient means acting between said rigid support and said movable core member to retain said cooperating bearing means in engagement with one another.

13. Apparatus according to claim 12, wherein said bearing means andsaid resilient means are so arranged that the direction in which said movable core member is urged by said resilient means is towards said rectangular core.

14. Apparatus according to claim 12, including nonmagnetic members attached to opposite sides of said movable core member and having surfacesextending parallel to the direction of movement of said movable core member, and wherein said rigid support is provided with surfaces extending parallel to said direction of movement, said surfaces of said non-magnetic members being engaged with said'surfaces of said rigid support and constituting said bearing means.

15. Apparatus according to claim 13, including metallic bean'ng blocks through which said. resilient means'act upon said movable core member, said blocks being in contact with said non-magnetic members.

16. Apparatus according to claim 12, including drive means for said movable core member carried by said rigid support, and loose coupling means interconnecting said drive means and said movable core member.

17. Apparatus according to claim 12, wherein said rigid support comprises a pair of castings fixedto opposite sides of said fourth limb of said rectangular core.

18. Apparatus according to claim 12, wherein said resilient means is adjustable to vary the force thereof acting upon said movable core member.

19. Apparatus according to claim 1, wherein the first limb and the second limb are split transversely of their longitudinal axes whereby the core is comprised of two parts, said two parts being secured to one another to provide said main flux path.

20. Apparatus according to claim 19, wherein the split in each limb is in the form of a V such that the surface area of the interface between the two parts at each split is substantially twice the cross sectional area of the respective limb. 

1. Electric arc welding apparatus comprising means for supporting a welding electrode adjacent to a workpiece to be welded, said support means permitting said electrode to be moved into contact with said workpiece and withdrawn therefrom for establishing an arc between the electrode and the workpiece; and power supply means for supplying current and voltage to said electrode and said workpiece, said power supply means being operable such that when said electrode is in contact with said workpiece thereby establishing short circuit conditions the voltage applied drops to substantially zero thereby to limit the maximum value of said short circuit current to a predetermiNed magnitude and when said electrode is withdrawn from said workpiece to establish said arc the voltage supplied increases from said zero value to a value less than the value under open circuit conditions to provide a welding current of predetermined magnitude, said power supply means further including adjusting means for selecting different values of said currents; said power supply means comprising a transformer which includes: a generally rectangular core having first and second limbs spaced from one another and third and fourth limbs spaced from one another and interconnecting adjacent ends of said first and second limbs, said first, second, third and fourth limbs providing a closed main flux path having no gaps therein; a primary coil which surrounds said first limb of said rectangular core; a secondary coil which surrounds said primary coil and is operable to produce said voltages and said currents; said adjusting means comprising a movable core member displaceable substantially rectilinearly in a direction parallel to said first limb between a first position in which said movable core member is substantially completely removed from a space defined between said primary and secondary coils and a second position in which said movable core member is within said space and extends at least between opposite ends of said secondary coil, and extension limbs magnetically coupling opposite ends of said movable core member to opposite ends of said first limb and said movable core member is in said space, said extension limbs and said movable core member providing through said space a flux leakage path whose reluctance is variable by varying the position of said movable core member; and means for inhibiting the flow of flux in said leakage path, at least when said movable core member is so positioned within said space as to provide low magnitudes of said welding current, to at least prevent the magnitude of said voltage under open circuit conditions from dropping by more than a predetermined amount; said transformer having no further primary and secondary coils and no further limbs and no further movable core members.
 2. Apparatus according to claim 1, in which said flux inhibiting means comprises a biassing coil arranged to produce in said flux leakage path a flux which opposes flux from the primary coil.
 3. Apparatus according to claim 2, including an auxiliary coil operable when current is drawn from said secondary coil to substantially cancel the effect of the flux from said biassing coil.
 4. Apparatus according to claim 3, wherein said biassing coil is connected to the primary coil and supplied with current therefrom, and the auxiliary coil is connected to the secondary coil and supplied with current therefrom.
 5. Apparatus according to claim 3, wherein said biassing coil is operable to produce a flux which maintains said voltage under open circuit conditions substantially constant for all positions of said movable core member.
 6. Apparatus according to claim 3, wherein said biassing coil is operable to produce a flux which increases the magnitude of said voltage under open circuit conditions the further into said space that the movable core member is positioned.
 7. Apparatus according to claim 1, wherein said flux inhibiting means comprises a gap between said movable core member and at least one of said extension limbs when the movable core member is fully inserted between the primary and secondary coils, said gap being so dimensioned that the magnitude of said voltage under open circuit conditions drops by not more than 10 percent when the movable core member is fully inserted into said space as compared to the magnitude of said voltage under open circuit conditions when the movable core member is completely removed from said space.
 8. Apparatus according to claim 1, wherein said flux inhibiting means comprises a gap between said movable core member and each of said extension limbs when the movable core member is fully inseRted between the primary and secondary coils, said gaps being so dimensioned that the magnitude of said voltage under open circuit conditions drops by not more than 10% when said movable core member is fully inserted into said space as compared to the magnitude of said voltage under open circuit conditions when the movable core member is completely removed from said space.
 9. Apparatus according to claim 1, wherein one side of said movable core member is tapered and an adjacent one of said extension limbs has an end surface presented towards said tapered side of said movable core member, said end surface being correspondingly tapered such that a gap of increasing width is provided between said end surface and said one side of said movable core member upon withdrawal of said movable core member from said space.
 10. Apparatus according to claim 1, wherein said movable core member has an end face presented towards a side face of one of said extension limbs, one of said faces being provided with a recess and the other of said faces being provided with a projection arranged for location within said recess when the movable core member is fully inserted into said space, said projection and recess providing for smooth adjustment of the magnitude of said short circuit current and said welding current when said magnitude is near to the minimum available and providing for reduction in magnetic forces between said movable core member and said extension limb when said movable core member is in the region of its fully inserted position.
 11. Apparatus according to claim 1, wherein at least one but not more than a small number of the turns of said secondary coil is located between said primary coil and the path of movement of the movable core member to provide for all positions of said movable core member relatively larger short circuit currents substantially without affecting the magnitude of said welding currents.
 12. Apparatus according to claim 1, including a rigid support fixed against movement relative to said rectangular core, means mounting said movable core member on said rigid support, co-operating bearing means provided on said rigid support and said movable core member, said bearing means permitting said movement of said movable core member, and resilient means acting between said rigid support and said movable core member to retain said co-operating bearing means in engagement with one another.
 13. Apparatus according to claim 12, wherein said bearing means and said resilient means are so arranged that the direction in which said movable core member is urged by said resilient means is towards said rectangular core.
 14. Apparatus according to claim 12, including non-magnetic members attached to opposite sides of said movable core member and having surfaces extending parallel to the direction of movement of said movable core member, and wherein said rigid support is provided with surfaces extending parallel to said direction of movement, said surfaces of said non-magnetic members being engaged with said surfaces of said rigid support and constituting said bearing means.
 15. Apparatus according to claim 13, including metallic bearing blocks through which said resilient means act upon said movable core member, said blocks being in contact with said non-magnetic members.
 16. Apparatus according to claim 12, including drive means for said movable core member carried by said rigid support, and loose coupling means interconnecting said drive means and said movable core member.
 17. Apparatus according to claim 12, wherein said rigid support comprises a pair of castings fixed to opposite sides of said fourth limb of said rectangular core.
 18. Apparatus according to claim 12, wherein said resilient means is adjustable to vary the force thereof acting upon said movable core member.
 19. Apparatus according to claim 1, wherein the first limb and the second limb are split transversely of their longitudinal axes whereby the core is comprised of two parts, said Two parts being secured to one another to provide said main flux path.
 20. Apparatus according to claim 19, wherein the split in each limb is in the form of a V such that the surface area of the interface between the two parts at each split is substantially twice the cross sectional area of the respective limb. 